1. Field
The invention relates to an instrument for inserting a bone anchoring element into a bone or a vertebra, wherein the instrument has a cutting member for cutting bone material surrounding the implantation site. Further the invention relates to a system of such an instrument and a bone anchoring element and in particular to a system of such an instrument and a polyaxial bone anchor. The instrument includes a shaft with an end portion for engaging the anchoring element and a cutting member connected to the shaft. The cutting member comprises an outer diameter that is substantially the same or larger than an outer diameter of a receiving part of a polyaxial bone anchor, such that the cutting member can cut bone material to provide the necessary space for the receiving part to be mounted after the bone anchoring element has been inserted into the bone. The instrument is particularly useful together with a bottom loading type polyaxial pedicle screw.
2. Description of Related Art
Various inserting instruments for inserting a screw member of a polyaxial pedicle screw are known. For example, U.S. Pat. No. 6,443,956 B1 describes a vertebral drill bit for forming a pathway through a pedicle into a vertebral body. The vertebral drill bit includes a cutting shank having a generally uniform diameter, an attachment head at one end of the cutting shank and a tip at the other end of the cutting shank. A flute is formed in the cutting shank. With the instrument, a hole is drilled through a pedicle in which the screw member a polyaxial pedicle screw can be inserted later. The cutting shank flares proximate to the attachment head to substantially match the diameter thereof. This flared portion and the corresponding flared portion of a sharp edge of the flute form a countersink in the cortical bone to receive an integral nut of conventional screws.
A drill which is adapted to impact and provide a countersink for a bone for orthopedic surgery is known from U.S. Pat. No. 8,029,509 B2. The drill has a head that has a modified egg shape and a plurality of cutting flutes. The drill is designed specifically to provide a countersink in the cortical bone at the implantation site so that a round headed screw or peg will not remain proud or project too far beyond the surface of the bone.
WO 2011/068516 A1 describes a base reamer to be used in connection with an implanted pedicle screw. A particular base reamer is selected to correspond to the specific implant base that will be implanted in a given pedicle.
Recently, bottom loading type polyaxial bone anchors, such as a polyaxial bone anchor known from US 2010/0234902 A1, have been utilized in such a manner that the bone anchoring element is inserted into the bone first and after insertion the receiving part is mounted to the head of the bone anchoring element. This is advantageous in certain applications in which the insertion of the polyaxial bone anchoring device consisting of the pre-assembled anchoring element and the receiving part is too difficult, for example, because of limited space or restricted accessibility of the implantation site.
When mounting the receiving part “in-situ” onto the head of the bone anchoring element, a problem might occur in that there is not enough space around the head of the bone anchoring element for placing the receiving part onto the head or for pivoting the receiving part relative to bone anchoring element.